LucasArts has said over and over that it does indeed contain spoilers.

As to what an "average fan" would know that's a completely subjective opinion. Some fans have hyperspace accounts and read the gossip/spy reports and news alerts at StarWars.com and Theforce.net every day, thus knowing just about everything about the new movie already.

Other "average fans" haven't even seen the teaser trailer yet.

So anyway, there you go.

But on the box itself it says vehicles, locations, characters, etc from Episode III. Right there on the box.

This game has spoilers. If you want to remain spoiler free, don't play it. Or if you must get it, do what I'm doing (once I get the game this weekend), and stick to multiplayer.